Surface Area Determination with a Laboratory Scale and Room Humidity from Single-Point BET Analysis of Physisorbed Water

For decades, nitrogen physisorption and the BET equation have been utilized for the characterization of the specific surface area of porous materials. Quantifying physisorbed nitrogen at cryogenic conditions is not instrumentally challenging nor relatively expensive, as liquid nitrogen is common to many chemical laboratories, and turnkey commercial physisorption units are within the budget of many research laboratories. However, a great simplification in instrumentation and a decrease in measurement cost can be achieved by physisorption experiments performed at room temperature and pressure. In this work, it was hypothesized that single-point BET analysis can be applied to physisorbed water at room pressure and temperature with humidity as the measure of relative pressure. That is, the surface area of porous materials can be determined with a lab balance to measure mass and a hygrometer to measure humidity at ambient temperature and pressure. The hypothesis was tested first in a thermogravimetric analyzer and second with a simple benchtop scale and a common hand-held hygrometer for six types of alumina beads, three with microporous and mesoporous pore size distributions and three predominantly mesoporous samples. The single-point BET equation is used to correlate the monolayer capacity of adsorbed water with the TGA mass loss and laboratory humidity. By correlating water monolayer capacity with N₂ BET surface areas, the effective cross-sectional area of H₂O was calculated to be approximately 20 Ų. This value was applied to the single-point BET analysis of humidity versus mass of samples made on the benchtop scale. The mean error of the benchtop balance measurements was 8.5%, suggesting the feasibility of using simple instrumentation under ambient conditions for surface area determination. It is in principle possible to extend the ambient humidity BET to other materials.